Er,Yb:YAB laser system

ABSTRACT

A laser system, the laser comprising a codoped Er,Yb:YAB gain medium, the gain medium within a resonator cavity. The laser system further comprising a pumping source, the pumping source having optical output directed towards the gain medium. The laser system further comprising a laser controller, the laser controller operating the pumping source.

TECHNICAL FIELD

The present disclosure relates in general to solid state laser systems.The disclosure relates in particular to Er,Yb:YAB based solid statelaser systems.

BACKGROUND INFORMATION

Lasers are used in a variety of applications. Lasers are used inmedical, manufacturing, military, and consumer applications. One lasertype, solid-state lasers, are based on crystalline or glass rods dopedwith ions. The doped rods are within in a resonator cavity and pumped toexcited states which decay emitting laser light.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to a laser system and methods ofoperating a laser system. The laser comprising a codoped Er,Yb:YAB gainmedium, the gain medium within a resonator cavity. The laser systemfurther comprising a pumping source, the pumping source having opticaloutput directed towards the gain medium. The laser system furthercomprising a laser controller, the laser controller operating thepumping source.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred methods and embodimentsof the present disclosure. The drawings together with the generaldescription given above and the detailed description of preferredmethods and embodiments given below, serve to explain principles of thepresent disclosure.

FIG. 1 is a block diagram of a laser system with a copdoped Er,Yb:YABgain medium, the laser system within a resonator cavity, a pumpingsource, the pumping source directed towards the gain medium, a heatspreader, the heat spreader in thermal communication with the gainmedium, and a laser controller, the laser controller operating the lasersystem.

FIG. 2A is a plan view of an embodiment of the laser system with anend-pumped design.

FIG. 2B is a plan view of an embodiment of the laser system wherein thepump source is a laser diode integrated on a baseplate.

FIG. 2C is a plan view of an embodiment of the laser system wherein theheat spreader is not within the clear aperture of the Er,Yb:YAB gainmedium.

FIG. 2D is a plan view of the laser system wherein an off-axis activeoptical q-switching is employed.

FIG. 2E is a plan view of the laser system wherein on-axis activeoptical q-switching is employed.

FIG. 3A is a graph of the absorption coefficient spectra of theπ-polarization and a-polarization states of the Er,Yb:YAB gain medium.

FIG. 3B is a graph of the emission spectra of the π-polarization stateof the Er,Yb:YAB gain medium.

FIG. 3C is a graph of the emission spectra of the 6-polarization of theEr,Yb:YAB gain medium.

FIG. 4A is an exploded perspective view of one embodiment of the heatspreader wherein the Er,Yb:YAB crystal is in thermal communication withan optically transparent window and a thermally conductive mount.

FIG. 4B is an exploded perspective view of another embodiment of theheat spreader wherein the Er,Yb:YAB similar to that shown in FIG. 4A,further comprising another thermally conductive mount in thermalcommunication with the gain medium.

FIG. 4C is an exploded perspective view of another embodiment of theheat spreader wherein the gain medium is sandwiched between twooptically transparent windows and thermally conductive mounts.

FIG. 4D is an exploded perspective view of one yet another embodiment ofthe heat spreader, similar to that shown in FIG. 4C, wherein anadditional thermally conductive mount is between the two opticallytransparent windows.

FIG. 4E is an exploded perspective view of the heat spreader wherein theheat spreader is not in contact with the clear aperture of the gainmedium.

FIG. 4F is cross-section view of another of the heat spreader, whereinthe heat spreader is not in contact with the clear aperture of the gainmedium.

FIG. 5A is a graph of a bias pump method operating the laser system.

FIG. 5B is a block diagram of the bias pump method steps illustrated inFIG. 5A

DETAILED DESCRIPTION OF EMBODIMENTS

Referring now to the drawings, wherein like components are designated bylike reference numerals. Methods and various embodiments of the presentinvention are described further hereinbelow.

FIG. 1 is a block diagram illustrating a preferred embodiment 10 of alaser system. Laser system 10 includes a copdoped Er,Yb:YAB gain medium12, gain medium 12 within a resonator cavity 14. Gain medium 12 is inthermal communication with a heat spreader 18. A pumping source 16directs output towards gain medium 12. A laser controller 20 operatesthe laser system, by controlling a pump driver 21, the pump driverdelivering current to pump source 16.

The Er,Yb:YAB gain medium is a yttrium (Y) aluminum (Al) borate (BO)crystal (a crystal commonly referred to as YAB) codoped with erbium (Er)and ytterbium (Yb). The gain medium is expressed through the presentdisclosure as Er,Yb:YAB. The Er,Yb:YAB gain medium is a negativeuniaxial crystal with 32 point symmetry. The codoped crystal isstoichometrically expressed Er,Yb %:YAl₃(BO₃)₄, where the percentage ofEr and Yb depend on the concentrations of the dopants in the hightemperature dipping seeded solution growth (DSSG) of the crystal usingthe flux method. In growth, Er and Yb ions replace Y sites within theYAB crystal. One nonlimiting example of a suitable dopant concentrationfor the present embodiment is Er at about 22 at. % and Yb at about 1.5at. %. Er doping concentrations can range from about 1% to about 25%. Ybconcentrations can range from about 0.5% to about 5%. Otherconcentrations of Er and Yb as well as various other doping ions can beincorporated within the YAB crystal. Doping ions, such as chromium (Cr),Iron (Fe) and gallium (Ga) can be incorporated within the YAB crystalduring growth, replacing Al sites.

The gain medium crystal can be cut on both an a-cut or a c-cutcrystallographic axes. The c-cut Er,Yb:YAB gain medium has a smallerabsorption cross-section and absorbs more efficiently than the a-cutEr,Yb:YAB gain medium. When cut and polished the Er,Yb:YAB gain mediumcan be shaped with a cross-section that is curved edges, with straightedges, such as a polygon, or combinations thereof. Further the Er,Yb:YABgain medium cross-section can change to any desired shape along itslength with abrupt or smooth transitions. In one embodiment theEr,Yb:YAB gain medium cross-section is circular with diameter about thesize of the expected pumping source illumination area.

Er,Yb:YAB gain medium 12 has absorption bands in the near-infrared (NIR)and stimulated emission bands at longer wavelengths bands in thenear-infrared. Specific absorption and emission bands are detailedfurther hereinbelow. Through the present disclosure the wavelengths usedto excite the gain medium are generally referred to as pumping radiationor near-infrared (NIR) short wavelength bands. The wavelengths emittedfrom the gain medium after excited state radiative decay are generallyreferred to as NIR long wavelength bands and called lasing emission,stimulated emission or laser pulse when the radiative decay is caused bystimulated emission. When optically pumped the Yb ions absorb NIR shortwavelength and efficiently transfer the absorbed energy to an excitedstate of the Er ions. Rapid non-radiative decay of the Yb excited statepopulates the upper level of the Er laser transition. The Er lasertransition can decay to its ground state by radiative decay bystimulated emission. Nonlimiting design factors for the Er,Yb:YAB gainmedium include the crystal shape, diameter, thickness, Yb dopingconcentration, Er doping concentration, Al replacement site doping,crystallographic axis cut, angular orientation with respect to pumppolarization, surface coating and surface shape. Such Er,Yb:YAB crystalsare commercially available from Voxtel, Inc. of Beaverton, Oreg.

Pumping source 16 outputs optical energy directed towards gain medium12. Pumping source 16 can be a lamp, led, laser diode, laser system, orother optical radiation source. Depending on the pumping source type,the optical output of pump source 16 can vary with respect towavelength, optical bandwidth, output angle, spatial intensity profile,angular intensity profile, polarization, or pump profile andcombinations thereof. Further a plurality of the pump sources can beemployed. The pumping source wavelength and optical bandwidth can bealtered based on the pumping source type and can be modified by, forinstance, dielectric coating, Bragg grating, or other such opticalfilter. Pump source 16 can be oriented to provide polarized, randomlypolarized or partially polarized optical radiation. When the light ispolarized or partially polarized, the polarization azimuth can be fixedat any angle or otherwise elliptically rotate.

Laser controller 20 operates and controls a pump driver 22, the pumpdriver delivers current to pump source 16. Pump source 16 can be drivenwith constant or modulated current profiles. A plurality of pump driverscan be employed for each of the pumping sources, when more than onepumping source is provided The current profile can be digital, analog,or combinations thereof. Specific modulation methods are discussed ingreater detail further hereinbelow.

A pump delivery optic 24 receives optical output from pump source 16 anddirects the pumping radiation towards Er,Yb:YAB gain medium 12. The pumpdirects light emitted by the pump source 16 such that gain medium 12 isilluminated in an area about corresponding with the desired stimulatedemission beam size. Pump delivery optics 24 can include refractive,reflective, diffractive, waveguides, spatial filters, optical filters,and combinations thereof.

In addition to directing the pumping radiation to the gain medium, thepump delivery optics can include spatial filtering and homogenizationoptics in order to provide the desired spatial illumination profile onthe gain medium. Nonlimiting factors in designing the pumping opticsinclude the number of the pumping sources, desired wavelength, opticalbandwidth, entrance angle, spatial intensity profile, angular intensityprofile, polarization, and combinations thereof. The pump radiationdirected by pump delivery optics 24 enter resonator 14, the Er;Yb:YABgain medium within the resonator.

Resonator 14 is defined by a first resonator surface 26 on an inputcoupler and a second resonator surface 28 on an output coupler. Theinput coupler transmits the pump radiation, allowing the pump radiationto enter gain medium 12. The input coupler at least partially reflectslaser radiation. Output coupler 28 at least partially reflects lasingradiation. In general the output coupler is the surface that allowslaser pulses to exit the resonator, although in some embodiment theinput coupler allows pulses to exit and is therefore both the inputcoupler and the output coupler. In practice, the input and outputcoupler surfaces can be defined on any surfaces of the optical-elementswithin the laser system, including the gain medium, so long as multiplestimulated emission resonates through the gain medium. Resonator 14 canincorporate end-pump, side-pump, ring-oscillator and other suchresonator designs. Further a plurality of the input coupler and theoutput coupler surfaces can be employed.

Within the resonator is a q-switch 30. Q-switch 30 can be an active orpassive device. Passive q-switching techniques include saturableabsorber and thin-film absorbers including bulk nonlinear saturableabsorbers, semiconductor saturable absorber mirrors (SESAMs), saturableabsorber output coupler and related devices. Active q-switching includeelectro-optic, acousto-optic, mechanically rotating or switching,microelectromechanical systems, and hybrid optical driven passiveq-switching. When the q-switch is active, the laser controller operatesthe q-switch directly or through a driver. Nonlimiting factors indesigning the q-switch, although such considerations are coupled to thelaser system design as a whole, include the required numerical aperture,maximum pulse frequency modulation, pulse duration, pulse width,recovery time, peak pulse power, modulation depth, damage threshold, andabsorption coefficient.

Er:Yb:YAB Gain medium 12 is in thermal communication with a heatspreader 18. Heat spreader 18 allows heat dissipation from Er,Yb:YABgain medium 12 when the laser system is operated at higher powers orotherwise allow temperature control of the Er,Yb:YAB gain medium. Heatspreader 18 can be part of a mechanical mount in thermal contact withthe gain medium on the crystal surfaces, perimeter, or both. The heatspreader can be a thermally conductive epoxy, solder, or other curablesubstance that can act as both an adhesive and allow thermal transport.Alternatively the heat spreader can be an optically transparent window,at one of the optically transparent window surfaces in contact with oneof the Er,Yb:YAB gain medium surfaces.

Thermal communication between the heat spreader and the Er,Yb:YAb gainmedium can be increased with thermal conductive epoxy, paste, solder,wettable metals such as indium, and techniques such as plating,conformal contact, optical contact, or diffusion bonding whenappropriate. Conformal contact refers to approximate surface matchingand can include adhesives. Optical contact refers to conformal surfaceshaping wherein each surface is optically quality, typically surfaceconformance better than about 1-2 nanometers, and contaminant free, suchthat intermolecular forces hold the surfaces together. Diffusion bondingis similar to optical contact except in addition to optical contact,heating and pressure processes reaching up to 60%-80% of the meltingtemperature to allow atomic diffusion of elements between the partsthereby forming a bond.

The heat spreader can be in thermal communication with a heat sink. Theheat sink or the heat spreader can be thermally controlled with passivecooling, active cooling, or combinations thereof. Passive cooling caninclude fins, rods, metal foam and other such structured surfaces. Suchfeatures can be placed on or within the heat spreader or a conductivemount, baseplate, or any heatsink which the heat spreader is also inthermal communication. Active cooling can includes fans, circulatingfluids or thermoelectric coolers.

Optionally a temperature monitor 32 can be placed on gain medium 12,heat spreader 18, pumping source 16, on any other element within thelaser system or in thermal communication with an element within lasersystem and combination thereof. Temperature monitor 32 can be athermistor and provide temperature feedback to laser controller 20.Laser controller 20 can then control operation of any active coolingdevices provided or otherwise alter operation of the laser system basedon previous characterization or concurrently measured laser performance.For instance concurrent measured performance can be done by integratingan optical detector 34.

Optical detector 34 can measure optical radiation from either the pumpsource, scattered laser radiation, or direct laser radiation andcombinations thereof. Optical detector 34 can be a PIN photodiode,avalanche photodiode, piezo based detector, or any other detectorcapable of fast energy pulses detection. One semiconductor materialcapable of detecting NIR is InGaAs. Either a single or plurality ofoptical detectors can be implemented for detecting optical radiationfrom the laser system. Detected laser radiation can provide feedback tolaser controller 20. Laser controller 20 can then control operation ofthe pump driver, or any other controllable device provided, based on theoptical feedback.

In addition to the optical components specifically referenced, a varietyof optical components can be implemented within the laser system, placedat any position between the pumping source and the laser output. By wayof example, an optical component 36 can be placed between Er,Yb:YAB gainmedium 12 and Q-switch 30 or an optical component 38 can be placedbetween q-switch 30 and output coupler 28. The optional opticalcomponents can be a single or plurality of refractive lenses, reflectivemirrors, diffractive surfaces, spatial filters, spectral filters,polarizers, attenuators and combinations thereof. Introduction of suchelements allows modification of the laser emission, nonlimiting examplesof which include resonator characteristics, beam shape, beam size,allowed modes, mode mixing, divergence, wavelength, spectral bandwidth,and polarization.

Referring to FIG. 2A, one embodiment of the present invention, a lasersystem 50A has codoped Er,Yb:YAB gain medium 12. Er,Yb:YAB gain medium12 has a first surface 12F and a second surface 12S. Gain medium 12resides within resonator cavity 14. Resonator cavity 14 is defined by afirst surface 14F and a second 14S, the resonator cavity surfaces alsobeing surfaces of components along a optical axis 54.

Here, the pumping source is a fiber coupled laser diode 54. Fibercoupled laser diode (LD) 54 has output directed towards Er,Yb:YAB gainmedium 12. In this embodiment, laser system 50A has an end-pump passiveq-switch design with fiber coupled LD 54 emitting a short wavelength NIRoptical radiation as along optical-axis 52 towards gain medium 12. Shortwavelength NIR optical radiation Xs is in the near-infrared absorptionband of Er,Yb:YAB gain medium 12. Fiber coupled laser diode 54 has aoptical fiber 56 terminating in a cylindrical ferrule 58. Ferrule 58mechanically fastened in 3-point contact mount 59, 3-point mount 59integrated within, or mechanically fastened to, a baseplate 61. Opticalfiber 56 terminates at the laser diode package, the laser controlleroperating the fiber coupled laser diode via a driver, the lasercontroller and the pump driver not shown in the present view.

Fiber coupled LD source 54 can comprise of one emission source or aplurality of emission sources. For instance the pump source can be asingle semiconductor laser diode element, a plurality of semiconductorlaser diode elements. The laser diode sources can be coupled into asingle optical fiber or a plurality of optical fibers all of which aredirected towards the Er,Yb:YAB gain medium. The optical fibers can besinglemode, polarization maintaining, or multimode. The optical fibercan include features such as mode mixing structures, bragg gratings,endcapping, coating, angled surface termination other such features.

Short NIR optical radiation Xs, represented as short dashed marginalrays, exit ferrule 58 according to the numerical aperture of opticalfiber 56, or by the numerical aperture as modified by optionalend-capping termination, towards the pump delivery optics. Here the pumpdelivery optics is an aspheric optic 60. Aspheric optic 60 has a firstsurface 60F and a second surface 60S. Optical radiation λ_(s) refractsat aspheric optic surface 60F, propagates through the aspheric lens andrefracts at second surface 60S, converging towards heat spreader 18 andgain medium 12.

Here, the heat spreader is an optically transparent window 62. Opticallytransparent window 62 has a first surface 62F and a second surface 62S.Here, heat spreader second surface 18S is in physical contact with gainmedium first surface 12F, thereby in thermal communication. Opticalradiation λ_(s) enters heat spreader 62 refracting at window firstsurface 62F. Optically transparent window 62 is a NIR transparent windowwith low absorption of optical radiation λ_(s). For sufficient thermalcontact with the Er,Yb:YAB gain medium, the optically transparent windowmay be bonded in conformal contact either with adhesives or bynon-adhesive contact techniques such as optical contact or diffusionbonded. The optically transparent window is connected to baseplate 61,the baseplate acting as a heat sink for the heat spreader and a mountingstructure for other optical components. In this configuration heatspreader 62 is within resonator 14 wherein window first surface 62F andfirst resonator surface 14F are the same surface. Window first surface62F is coated with a short NIR wavelength anti-reflective (AR-coating)coating and a long NIR wavelength high-reflectance coating (HR-coating),thereby allowing the pumping emission into the gain medium whileconfining stimulated emission from the gain medium within the resonator.When the heat spreader is in optical or diffusion contact with the gainmedium and the heat spreader is are made from gain medium index-matchingmaterial, for instance sapphire, the short wavelength optical radiationenters the gain medium with substantially no loss from reflectance.Various embodiments and configurations of the heat spreader arediscussed in detail further hereinbelow

The short wavelength optical radiation enters the Er,Yb:YAB gain mediumand the gain medium absorbs the short wavelength radiation. Theabsorption of the short wavelength optical radiation is dependent on thewavelength, pump polarization, crystallographic cut, stoichiometry, andorientation, the length and the illuminated cross-section of theEr,Yb:YAB gain medium. Er,Yb:YAB gain medium second surface 12S canoptionally have a short wavelength HR-coating to reflect any unabsorbedshort wavelength radiation.

The absorbed short wavelength radiation populates the upper energystates of the Er,Yb:YAB gain medium. Initially a broad spectrum of longwavelength radiation is emitted. Some of the long wavelength emissionpropagates to the q-switch, here the q-switch is a saturable absorber64. Saturable absorber 64 has first surface 64F and second surface 64S.Saturable absorber 64 is a material that absorbs optical radiation up toa saturation point. Upon saturation the saturable absorber loses itsability to absorb, becoming suddenly transmissive. One suitablesaturable absorber is Cobalt Spinel (Co:Spinel). Cobalt spinel absorbsfrom about 1100 nm to about 1600 nm. Alternatively a saturable Braggabsorber or reflector could be used.

As the gain medium upper energy levels populate and emit a longerwavelength radiation λ_(L), the saturable absorber saturates suddenlybecoming a low loss medium and transmits long wavelength radiationλ_(L). The long wavelength emission propagates to a output coupler 66.Output coupler 66 has a first surface 66F and a second surface 66S.Here, absorber first surface 66F is curved such that reflected radiationpropagates back through the resonator such that the reflected longwavelength radiation beam size is about the same size as the shortwavelength beam size illuminating the Er:Yb:YAB gain medium. Firstsurface 66F has a partially reflective coating wherein the partiallyreflective coating reflects a discrete band of long wavelengthradiation. The discrete wavelength band reflects back to Er,Yb:Gainmedium 12 causing stimulated emission of populated energy states in theEr,Yb:YAB gain medium.

Lasing occurs when the Er,Yb:YAB medium is pumped sufficiently to causepopulation inversion, and losses become suddenly low due to saturationof the saturable absorber, and the roundtrip gain caused by simulatedemission within the resonator exceed losses. The energy stored in thepopulation inversion is emitted in a fast, intense laser pulse. Thelaser pulse transmits through output coupler second surface 36S. Afterpulse emission, and depletion of the Er,Yb:YAB energy states, thesaturable absorber recovers to high-loss state and another pulse iseventually emitted, depending on continued or modulated pump currentprovided to the pumping source.

Here, the optional optical monitor provided is an avalanche photodiode(APD) 72. APD 72 detects a scattered light 74 from the intense laserpulse. APD 72 is placed at the periphery of the laser system such thatthe APD is not directly in the optical path of the laser system. Placingthe APD at the periphery allows for detection of light scattered fromoptical elements during pulse emission. Using an APD allows detection ofoptical radiation at low levels. If scattered light is sufficient a PINor other optical detector can be used. APD 72 may connect to the lasercontrol and provide temporal feedback of the laser pulses. The opticalmonitor can provide temporal feedback of the pulse emission to the lasercontroller, which in turn can alter operation of the laser system basedon the feedback.

FIG. 2B illustrates a laser system 50B. Laser system 50B is similar tothat shown in FIG. 2A, except here the pumping source is an integratedlaser diode 80. Laser diode 80 is directly mounted on baseplate 61.Pumping radiation λ_(S) is emitted and coupled into the beam deliveryoptics. Here the beam delivery optics include an optional mode filteringoptic 86 and aspheric lens 60. Mode filtering optic 86 may includeoptical fiber, anamorphic optic, or other such beam shaping optic toallow modification of transverse modes. Such mode filtering optics allowfiltering of single-mode and multimode output pumping radiation in orderto achieve the desired beam profile. For instance, if the pumpingradiation emits an anamorphic beam, whether multimodal or single mode,the mode filtering optic can homogenize or strip the modes to provide asingle or multimodal beam structure in order to provide even circularillumination onto gain medium 12. If the mode filtering optic includes amultimodal optical fiber, mode scrambling can be implemented by pinchingoptical fiber in various locations in order to provide a high ordertransverse mode homogenized illumination.

Here a laser controller 82 is shown integrated on baseplate 61. Thelaser controller providing pump current to laser diode 80. Lasercontroller 82 is also in electrical connection with photodetector 72,temporally monitoring lasers pulses.

FIG. 2C is a plan view of a laser system 50C, another embodiment of thepresent disclosure. Laser system 50C is similar to that shown in FIG.2A, except here the heat spreader is a mechanical mount 90, themechanical mount in thermal communication with the side and optionallythe perimeter of gain medium 12, the mechanical mount not in contactwith the clear aperture. The clear aperture being the portion of thegain medium surface through which pumping or lasing radiation isintended to propagate. The clear aperture can be centered, off-center,symmetric and asymmetric with respect to the Er,Yb:YAB gain medium. Forinstance the pump radiation can be directed to an outer edge of theEr,Yb:YAB gain medium, the outer-edge being closer to bulk thermalspreader material to allow faster thermal transport.

In laser system 50C, first resonator surface 14F is Er,Yb:YAB gainmedium first surface 12F, first surface 12F having an AR coating for theshort wavelength radiation and an HR coating for the long wavelengthradiation. Alternatively, the heat spreader can be a thermallyconductive epoxy, solder-based, or combinations thereof. The heatspreader can be thermally conductive epoxy, the thermally conductiveepoxy acting as both the heat spreader and mounting adhesive bonding thegain medium to the baseplate. The gain medium can be metalized onsurfaces outside the clear aperture and any edges, then solder mountedonto baseplate 61.

FIG. 2D is a plan view of a laser system 50D, similar to that shown inFIG. 2A, except here saturable absorber 64 is additionally controlledwith another radiation source. A saturable absorber optical driver 100has a optical radiation λ_(SA) directed towards saturable absorber 64.Saturable absorber optical driver 100 can be an LED, LD, laser or othersuch radiation source. The saturable absorber optical driver wavelengthmust be within the spectral absorption region of the saturable absorber64.

The laser controller operates saturable absorber optical driver 100,allowing increased control of when saturable absorber 64 becomestransmissive. Optical radiation λ_(SA) from the saturable absorberoptical driver can be held constant or modulated. In constant currentmode, intensity of optical output changes the frequency and pulse energyoutput from the laser system. Increased optical driver output radiationincreases pulse emission frequency and lowers pulse energy. Modulatingthe saturable absorber optical driver, in coordination with driving thepump source allows increased deterministic control over saturation ofthe saturable absorber. Such a hybrid passive/active q-switching allowscontrol over the pulse energy and temporal pulse emission.

FIG. 2E is a plan view of a laser system 50E, similar to that shown inFIG. 2D, except here a spectral beam combiner 102 directs from saturableabsorber optical driver 100 to saturable absorber 64. Spectral beamcombiner 102 is between Er,Yb:YAB gain medium 12 and saturable absorber64. Here the spectral beam combiner is a cube and has a spectral filteron the 45 degree interface 104, the spectral filter transmitting longNIR wavelength emission λ_(L) and reflecting saturable absorber driveremission λ_(SA). Having an on-axis spectral beam combiner allows bettermatching of the cross-section of the saturable absorber that isilluminated.

FIG. 3A is a graph 110 showing the polarized absorption coefficientspectra of Er,Yb:YAB from about 900 nm to about 1025 nm. A π-polarizedabsorption coefficient spectra 112 has a relatively flat response with asubstantially flat response region 114 at about 940 nm. A σ-polarizationabsorption coefficient spectra 116 has absorption with spectralabsorption peak 118 at about 976 nm with a full width half max (FWHM)120 of about 17 nm. As aforementioned, the amount of optical pumpingoutput absorbed depends on the pumping wavelength, pumping polarization,and thickness of the Er,Yb:YAB gain medium. When pumped witha-polarization at about 976 nm the thickness for the c-cut Er,Yb:YABgain medium can be from about 100 to about 200 microns (μm) due to thelarge absorption coefficient of about 184 cm⁻¹. To ensure pumping atabout 976 nm with a laser diode based pumping mechanism, the pump can bewavelength stabilized by either integrating a distributed Braggreflector or by controlling the temperature of the device. When pumpedwith the π-polarized state, the thickness of the Er,Yb:YAB crystal hasto be thicker relative to the σ-polarization thickness to achievesimilar absorption, but the pumping wavelength and variations thereofduring operation have less effect, especially when pumped in flatresponse region 114 at about 940 nm.

FIG. 3B and FIG. 3C show the Er,Yb:YAB gain medium emission spectra fromabout 1450 nm to about 1500 nm for a specific average pump power. FIG.3A is graph 110B showing π-polarization emission spectra 122. Emissionspectra 122 has multiple emission peaks. FIG. 3C is a graph 110C showingσ-polarization emission spectra 124. Emission spectra 124 has multipleemission peaks with a strong peak 126 at about 1530 nm. Asaforementioned partially reflecting coating design on the first surfaceof the output coupler can be tuned to provide spectral emission at anyof the emission wavelengths, the efficiency of the laser increasing whenthe partially reflecting coating is designed for emission peaks, forexample 1530 nm, and the efficiency also increasing when the Er,Yb:YABcrystal orientation is oriented to preferred polarization angles.

The Er,Yb:YAB gain medium absorbs radiation in substantially similarbands as shown in FIG. 3A and FIG. 3B and the pumping source can providepumping radiation at those wavelengths. For example the pumping sourcecan be a 1470 nm laser diode, with the output coupler having a partiallyreflective coating at a wavelength longer than 1470 nm. Implementingsuch a pumping source with pumping radiation in about proximity to thelasing emission wavelength reduces the amount of heat produced relativeto pumping at shorter wavelengths.

The heat spreader allows heat transfer from the gain medium when thelaser system is operated at high power. When excited states releaseenergy in nonradiative decay the energy is released as phonons. Whenoperating at high powers, for instance above about 1 watt (W) averagepower with 100% duty cycle, heat buildup on the gain medium can causethermal lensing effects and possibly cause damage to the crystal.Thermal lensing effects can lead to an instable resonator, limitingperformance and power. In general the heat spreader and any heat sink inthermal communication with the Er,Yb:YAB must be able to handle the heatgenerated in the crystal by the pump source. Heat generation depends onthe pump wavelength and the laser emission wavelength. Pump power toheat conversion can be from about 1% to about 45% of pump power. Theheat spreader embodiments described below allow high average poweroperation.

FIG. 4A shows embodiment of a heat spreader 150. Er,Yb:YAB gain medium12 is attached to a optically transparent window 152. Opticallytransparent window 152 has a first surface 152F and a second surface152S. Optically transparent window 152 can be made from sapphire,yttrium aluminum garnet, YAB, or other optically transparent materialsin the NIR. Preferred material for the optically transparent windowbased on refractive index matching, coefficient of thermal expansion(CTE) matching, and thermal conductivity. The first surface of gainmedium 12 is in thermal communication with optically transparent window152 in either conformal, optical contact, or diffusion bonded.

In this example, optical window 152 has a diameter larger than thediameter of gain medium 12. The surface area exposed allows thermalcontact to conductive mount 156. Conductive mount 156 has first surface156F and second surface 156S with thru-hole 158. The diameter ofthru-hole 158 is at least as large as the diameter of gain medium 12.Optically transparent window 152S is attached to conductive mount firstsurface 156F such that they are in thermal communication. Conductivemount 156 can be made from thermally conductive material such as metals,ceramics, and composite materials. Nonlimiting examples of such materialinclude copper, aluminum, aluminum nitride, and graphite/graphene/metalembedded polymers.

The method of attaching or fastening the optically transparent window tothe conductive mount can be mechanical or adhesive based, depending onthe materials used and expected operation power. For instance when theconductive mount and the transparent window have dissimilar CTE,clamp-based mechanical fastening, and holding two conformal surface incontact can be implemented. Thermal pastes or flexible metals such asindium can be applied to increase thermal communication. Likewiseadhesives with flexibility can be implemented. When adhesives areimplemented they are preferably thin or otherwise thermally conductiveto allow appropriate heat transfer. For instance thermally conductiveepoxies, films and tapes can be implemented.

The thermally conductive mount is attached to either the baseplate orother rigid mounting structure which can include passive or activecooling. The baseplate can be either mounted to another conductive heatsink for high power operation, or with sufficient surface area to allowenvironmental heat transfer. In one embodiment a water channel can beimplemented within the perimeter of conductive mount 76. Channels can bedesigned for laminar or turbulent flow. Alternatively a thermoelectriccooler (TEC) can be implemented either in connection to the baseplate orin direct contact with the conductive mount. Addition of thermistorsprovide temperature feedback and allow controlled operation of activecooling techniques, by for example, the laser controller.

Heat spreader embodiment 152 shows one preferred orientation whereinpumping radiation enters gain medium 12 from the first surface.Alternatively the orientation can be reversed. Further, conductive mount152 can be placed such that conductive mount second surface 156S is inthermal communication with optically transparent window first surface152F. When oriented in such a manner thru-hole 158 can be sized smallerthan the diameter of gain medium 12, still sufficiently large to allowthe pump radiation to pass unobstructed.

FIG. 4B shows a heat spreader embodiment 150B. Heat spreader 150B issimilar to the heat spread as that shown in FIG. 4 with the addition ofthermally conductive mount 160. Thermally conductive mount 160 has afirst surface 160F, a second surface 160S, and a thru-hole 162. Secondsurface 160S is in thermal communication with optically transparentwindow 152F. Here, thru-hole 162 can be sized smaller than the diameterof gain medium 12, still sufficiently large to allow the pump radiationto enter through the thru-hole.

FIG. 4C shows a heat spreader embodiment 150C. Heat spreader 150C issimilar to that shown in FIG. 4B with the addition of a opticallytransparent window 166. Transparent window 166 has a first surface 166Fand a second surface 166S. The second surface of gain medium 12 inthermal communication with transparent window first surface 166F.Transparent window second surface 166S is in thermal communication withconductive mount first surface 156F. Here, thru-hole 158 can be sizedsmaller than the diameter of gain medium 12, sized sufficiently large toallow lasing radiation to pass unobstructed.

FIG. 4D shows yet another heat spreader embodiment. A heat spreader 150Dhas that shown in FIG. 4C with the addition of a thermally conductivemount 170. Thermally conductive mount 170 has a first surface 170F and asecond surface 170S. Here thermally conductive mount 170 is the samethickness or thinner than gain medium 12. Second surface 152S ofoptically transparent window 152 is in thermal communication withconductive mount first surface 170S. When thermally conductive mount 170is the same thickness as gain medium 12, then second surface 170S isalso in thermal communication with optically transparent window firstsurface 166F.

FIG. 4E shows a heat spreader embodiment 150E, wherein the heat spreaderis not in contact with the gain medium clear aperture. Here a conductivemount 182 has a open diameter 184, the diameter at least as large asgain medium diameter 12D. Gain medium 12 resides within heat spreader182, fitting within diameter 184. Gain medium 12 is in thermalcommunication with conductive mount 182 by physical contact withretainer lip 186. Retainer lip 186 has an inner diameter 188 at least aslarge as the gain medium clear aperture 12C. The gain medium can besecured by mechanically, for instance with a retaining ring or with anadhesive. The thermal communication can be increased with addition ofthermal past, thermal epoxy, solder, wettable metal such as an indium,or other such filler.

FIG. 4F shows a heat spreader embodiment 150F, another of the heatspreader embodiments where the heat spreader is not in contact with thegain medium clear aperture. Here, heat spreader 190 is a thermallyconductive material, deposited when viscous, then cured. Two nonlimitingexamples include thermally conductive epoxy and solder. In thisembodiment the gain medium resides with a V-groove 62V, the V-groovemachined within baseplate 62. The thermal epoxy is deposited and inthermal contact with the base of gain medium 12, not in contact withclear aperture 12C. If increased thermal management of the gain mediumis required heat spreader 190 can be deposited to encompass the entiregain medium, not deposited within the clear aperture.

FIG. 5A and FIG. 5B illustrates a bias method of operating the lasersystem of the present disclosure to reduce pulse to pulse jitter. Ingeneral, pulsed laser performance metrics include average power, peakpulse power, pulse energy, pulse duration, and pulse frequency. Theaverage power is the product of pulse energy and repetition rate. Peakpulse power is the product of the pulse duration and pulse energy, bothof which depend on the detailed pulse shape. Pulse frequency depends onthe pumping mechanism, modulation, and q-switch device employed. Forpassive q-switching, high frequency pulse rates can be increasing pumpintensity, but pulse-to-pulse jitter or timing jitter occurs due toinstability of q-switching. Instable q-switching arises from a varietyof factors which include residual population inversion in the gainmedium, recovery of the saturable absorber, thermal effects of theEr,Yb:YAB gain medium, and pump source fluctuation.

Timing jitter is inversely proportional to the slope the populationinversion density as it is pumped to a lasing state, or within a lasingwindow. Increasing the population density slope, decreasespulse-to-pulse jitter. One method of increasing the slope is throughcontrol of the pumping source and pump profile.

FIG. 5A is of a graph 250A, depicting a current pump profile 250A.

FIG. 5B is a block diagram 250B, depicting the bias method steps asillustrated in FIG. 5A. During operation the pump source receives pumpcurrent from the pump driver, the pump driver controlled by the lasercontroller. The pump current determines the amount of short wavelengthNIR optical radiation emitted by the pump source. The pump current has apump current profile 252. Pump current profile 252 is held at an initialcurrent I_(i) such that little or no optical radiation is not emittedfrom the pump source. A LD for example emits little to no opticalradiation when operated at or below the laser threshold level. CurrentI_(i) can be at zero or any intermediate value about below the emissionthreshold of the pump source, thereby providing a pump source bias 254.

When a pulse is desired, the laser controller causes a current increase256 to a bias pump current I_(B). Bias pump current I_(B) causesincreased population density within the Er,Yb:YAB gain medium, but isbelow a lasing threshold current I_(L), lasing threshold currentsufficient to cause pulse emission if applied continuously. Bias pumpcurrent IB is applied for a bias duration τ_(B). Bias duration TB islong enough to cause increased population inversion density at about apredictable level.

A pump pulse 260 is delivered via a sudden increase in the current, pumppulse greater than a lasing threshold current I_(L). The pump pulsecauses a sudden increase in population inversion density, resulting in alaser pulse emission 262. The sudden increase causes populationinversion density slope to be high, shortening the temporal lasingwindow.

The pump current is reduced at point 266 after the emission of the laserpulse, down to after pulse current I_(AP). After pulse current I_(AP)can be at about any value between the zero and the bias current,depending on when the next desired pulse is desired and the residualpopulation inversion density. With the about predictable populationinversion level and steep population inversion slope, the lasing windowis shortened and pulse emission about temporally predictable loweringpulse to pulse jitter in continued operation.

The bias-method of operating the laser system can be implemented in avariety of ways. For instance, when continuous operation of the lasersystem is desired with identically temporally spaced laser pulses, thesame pump current profile can be implemented. Alternatively a pluralityof pump profiles can be used to cause varying delays between pulses. Thepump profiles can be altered by the pump source bias applied, the biaspump current, or the pump pulse and durations of each. In one embodimentthe pump profile can comprise two pump sources with two differentwavelengths, for example 940 nm and 976 nm. Pump current profile 250A isshown as a step-function, in practice the pump current can be driven asshown, as a non-linear continuous function, or as a series of pulses ofvarying amplitude and pulse width.

The pump profile can be constructed from one modulated source or aplurality of pump sources. For instance one pump source can provide thebias pump current and the pump pulse by modulation of the current of theone pump source. Alternatively, a plurality of pump sources can beimplemented with one pump source providing the bias pump current and asecond pump source providing the pump pulse. Further a combination aplurality of bias pump current sources and a plurality of pump pulsesources can be provided.

The present embodiments and methods described in the present disclosureinvention have a variety of useful applications. For instance the lasersystem can utilized in any general laser application. In particular thelasing emission bands are in about the so-called “eye-safe” region andcan be utilized in applications in areas in which intense laser pulsesare normally not permitted. Applications include machine vision inmanufacturing processes, robotics machine vision including autonomouscar guidance, range finding, LIDAR/LADAR applications, biomedicalapplication, target designation, and other such applications.

From the description of the present disclosure provided herein oneskilled in the art can manufacture apparatus and practice the methodsdisclosed in accordance with the present invention. While the presentinvention has been described in terms of particular embodiments andexamples, others can be implemented without departing from the scope ofthe present invention. In summary, the present disclosure abovedescribes particular embodiments. The invention, however, is not limitedto the embodiments described and depicted herein. Rather, the inventionis limited only by the claims appended hereto.

What is claimed is:
 1. A laser system comprising: a codoped Er,Yb:YABgain medium, the gain medium within a resonator cavity; a pumpingsource, the pumping source having optical output directed towards thegain medium; a laser controller, the laser controller operating thepumping source; and a heat spreader, the heat spreader in thermalcommunication with the gain medium through a surface wherein the pumpsource has optical output incident.
 2. The laser of claim 1, wherein thelaser is actively Q-switched, the active q-switch controlled by thelaser controller.
 3. The laser of claim 1, wherein the laser ispassively Q-switched.
 4. The laser system of claim 1, wherein the laserproduces greater than 1 Watt (W) average power.
 5. The laser system ofclaim 1, wherein the laser produces greater than produces great than 5kW peak pulse power.
 6. The laser system of claim 1, wherein the heatspreader is an optically transparent window about optically indexmatched to the Er,Yb:YAB gain medium and having no etalon effect.
 7. Thelaser system of claim 6, wherein the transparent window is sapphire. 8.The laser system of claim 1, wherein the laser controller prepumps theEr,Yb:YAB gain medium.
 9. The laser system of claim 8, wherein theduration of the pulse pumping is shorter than the upper-state lifetimeof the gain medium.
 10. The laser system of claim 8, wherein the pulsepumping is used to increase laser pulse energy.
 11. The laser system ofclaim 8 wherein the pulse pumping is used to increase average laserpulse power.
 12. A laser system comprising: a codoped Er,Yb:YAB gainmedium, the gain medium within a resonator cavity; a pumping source, thepumping source having optical output directed towards the gain medium; aheat spreader in thermal communication with the gain medium; a lasercontroller, the laser controller operating the pumping source; and aphotodetector, the photodetector detecting emission of the opticalpulse, the photodetector providing temporal feedback of the time of thelaser pulse output.
 13. The laser system of claim 12, wherein the laserproduces greater than 1 Watt (W) average power.
 14. The laser system ofclaim 12, wherein the laser produces great than 5 kW (Q) peak pulsepower.
 15. The laser system of claim 12, wherein the laser controlleradjusts operating conditions of the laser to reduce jitter.
 16. Thelaser system of claim 12, wherein the photodetector is a photodiode. 17.The laser system of claim 16, wherein the photodiode is InGaAs.
 18. Thelaser system of claim 12 wherein the photodetector is an avalanchephotodiode (APD).
 19. The laser system of claim 12, wherein the heatspreader is an optically transparent window about optically indexmatched to the Er,Yb:YAB gain medium and having no etalon effect. 20.The laser system of claim 19, wherein the transparent window issapphire.
 21. The laser system of claim 12, wherein the pumping sourceemission is about 976 nm.
 22. The laser system of claim 12, wherein thepumping source is a fiber coupled laser diode.
 23. The laser system ofclaim 12, wherein the laser controller operates an active thermalcooler, the thermal cooler in thermal communication with the Er,Yb:YABgain medium.
 24. The laser system of claim 12, wherein the lasercontroller modifies the pump current based on temporal feedback from thephotodiode.